Here, at Fort Craven, it was desired to find whether an enemy’s submarine boat could creep in, below the surface, find the mine, whose location was already known through spies, and effectively cut the firing wire.  If this could be done, then, in war-time, it might be that the sergeant at the wall-board would press the button in vain.  No explosion would follow.  With the current thus cut off, the officer bending over the white screen would not see the miniature reproduction of the destruction of the enemy’s battleship.

A submarine torpedo boat, coming into a harbor underneath the surface, is not pictured on the white table under the camera obscura.  So it was desired to see whether Eph could come in, knowing the exact locations of each of the four dummy mines, and quickly cut the firing electric wires.  If this could be done, the Army would have to revise its method of firing such submarine mines by means of the camera obscura detection.

As Eph listened to the explanation his mind began to revolve plans rapidly whereby he hoped to succeed in cutting the mine wires.

“You will keep sufficiently below the surface, too, Mr. Somers,” continued Major Woodruff.  “We do not want you so close to the surface of the water that a ripple would show on the camera obscura table.  You cannot, of course, rise and use your periscope to see where you are.  Even the periscope would betray you.”

The “periscope” is a device also of the nature of a camera obscura.  In the case of the periscope a narrow metallic tube is thrust above the water and the shutter turned about, reflecting all the scene about on a white-covered table in the boat’s cabin.

“I think I can beat you, Major,” smiled Eph.

“I certainly hope you can,” replied Major Woodruff.  “That is what we want to see today.  We shall watch closely, too, and see whether any plan can be devised for beating a submarine torpedo boat at its own game.”

Lieutenant Kline was to remain on board the “Spitfire,” both in order to watch the work and to give Eph any instructions that might be necessary in order to make the tests more conclusive.

“If you will come along with me, then, Mr. Benson,” suggested Major Woodruff, “I will put you ashore on the neck.  On the way over I will give you your instructions.”

As the tug came alongside again Jack followed the major over the gang plank to the deck of the other craft.

“Good-bye, Captain Somers,” called Jack, laughingly.  “Give a fine account of yourself as an enemy of the United States!”

“Oh, you—­” began Eph, flaring red, but wisely cutting his speech short.

On the way over to the strip of land known as the “neck” Major Woodruff managed to make his instructions wholly clear to young Benson.

“Now, you know what to watch for, and what observations, to report to me,” finished the major of engineers, as the tug came to a stop.  A small boat was lowered, and, in this, Captain Jack Benson was put on the desolate shore.